Orange Walnut Shortbread Cookies

In large bowl, combine powdered sugar and margarine. Cream until light and fluffy. Add egg yolk. Blend well. Lightly spoon flour into measuring cup. Level off. Add flour and cornstarch. Stir until mixture forms a smooth dough. Add walnuts and 1 tablespoon grated orange peel. Knead to blend. Shape into 1 inch balls. Place 2 inches apart on ungreased cookie sheets. Flatten with glass dipped in sugar. Bake at 350 for 8-13 minutes until lightly browned and set. Remove from cookie sheets to cool.

In small bowl, combine powdered sugar, butter, orange juice and 1 teaspoon grated orange peel. Beat until smooth. Frost cooled cookies.